The present inventors disclosed a laminated sheet (laminated plate) which has a thickness of 3.4 mm or below and which includes a polypropylene resin sheet containing a foaming agent and a thickness of from 0.5 to 1.4 mm; and an aluminum alloy sheet being laminated on each of the both sides of the polypropylene resin sheet and having a thickness of from 0.05 to 1.0 mm in Japanese Patent No. 4559513. The laminated sheet is subjected to cold forming and then heating to decompose the foaming agent to give a composite formed article which includes a foamed polypropylene resin (expanded) sheet; and an aluminum sheet laminated on both sides of the foamed resin sheet. The resulting composite formed article is used in a variety of applications.
The laminated sheet employs the polypropylene resin sheet having poor cold formability if used alone; in combination with an aluminum alloy sheet having a small thickness and thereby having further poor formability, but contrarily has significantly improved cold formability and is easy to form into a complicated shape. In addition, the laminated sheet also excels in form stability after forming. For these reasons the laminated sheet is advantageously usable as materials for providing formed articles (composite formed articles) having a large area and having a complicated shape, such as automotive panels and parts.
Recent automotive panels and parts should be used in environments at higher temperatures, and along with this requirement, composite formed articles to be used in these applications should have higher heat resistance.
A possible solution to provide a composite formed article having improved heat resistance is a method for producing a multilayer article using a polypropylene resin sheet having a high melting point.
However, a composite formed article obtained by using such a polypropylene resin sheet having a high melting point often suffers from delamination between the foamed polypropylene resin sheet after expansion and the aluminum alloy sheet.